In a process for manufacturing semiconductor devices or liquid crystal displays, a diluted solution obtained by diluting a chemical liquid, such as aqueous ammonia, with ultrapure water is used as a cleaning liquid for cleaning silicon wafers or glass substrates for liquid crystal panels. As a method of producing the diluted solution, there is known a method of adding a minute amount of a chemical liquid to ultrapure water supplied to a point of use, and various proposals have been made therefor.
For example, Patent Literature 1 proposes a method of adding a chemical liquid in a chemical tank to a pipe that carries ultrapure water by using a chemical liquid supply pump. In this method, electric conductivity of ultrapure water to which the chemical liquid has been added (diluted solution of chemical liquid) is measured, and based on the measured value, the amount of the chemical liquid to be added is adjusted.
Patent Literature 2 proposes a method of adding a chemical liquid by using a plurality of small pipes equipped with valves and connected in parallel between a circulation pipe that circulates ultrapure water and a feeder of the chemical liquid. In this method, the pressure at the time of adding the chemical liquid to the circulation pipe is adjusted to a certain fixed value, and the amount of the chemical liquid supplied to the circulation pipe is controlled in accordance with the number of the valves that are opened.
Furthermore, Patent Literature 3 proposes a method of adding a chemical liquid by using a chemical supply pipe that connects a cleaning liquid supply pipe to a chemical liquid storage container. In this method, the amount of the chemical liquid added from the chemical supply pipe to the cleaning liquid supply pipe is controlled by adjusting the pressure of gas supplied to the chemical liquid storage container based only on the flow rate of the cleaning liquid in the cleaning liquid supply pipe.